The shocking development follows the news late last week that traces of morphine had been found in samples provided by five horses, Estimate among them. According to the Racing Post, it is thought the positive drug tests may have been a consequence of eating contaminated feed.

Estimate, a 5-year-old daughter of Monsun trained by Sir Michael Stoute, was presented with the Cartier Stayer Award in November.

In a statement reported by the Racing Post, John Warren, the Queen's bloodstock and racing advisor, said: "On Thursday July 17 the British Horseracing Authority announced that a number of post-race samples, obtained from recent race meetings, had been found to indicate the presence of morphine, which is a prohibited substance on race days. Five horses, under the care of various trainers, were affected.

"I can confirm that one of those horses was Estimate, the 5-year-old filly trained by Sir Michael Stoute and owned by The Queen. Initial indications are that the positive test resulted from the consumption of a contaminated feed product.

"Sir Michael is working closely with the feed company involved to discover how the product may have become contaminated prior to delivery to his stables.

"As the BHA investigates this matter, including potential links between the different cases, Sir Michael continues to offer his full cooperation. There will be no further comment until the BHA announces its considered findings. Her Majesty has been informed of the situation."